Large-Size CH3NH3PbBr3 Single Crystal: Growth and In Situ Characterization of the Photophysics Properties

Effective Piezo-Phototronic Enhancement of Flexible Photodetectors Based on 2D Hybrid Perovskite Ferroelectric Single-Crystalline Thin-Films

2D hybrid perovskites are very attractive for optoelectronic applications because of their numerous exceptional properties. The emerging 2D perovskite ferroelectrics, in which are the coupling of spontaneous polarization and piezoelectric effects, as well as photoexcitation and semiconductor behaviors, have great appeal in the field of piezo-phototronics that enable to effectively improve the performance of optoelectronic devices via modulating the electro-optical processes. However, current studies on 2D perovskite ferroelectrics focus on bulk ceramics that cannot endure irregular mechanical deformation and limit their application in flexible optoelectronics and piezo-phototronics. Herein, we synthesize ferroelectric EA₄ Pb₃ Br₁₀ single-crystalline thin-films (SCFs) for integration into flexible photodetectors. The in-plane multiaxial ferroelectricity is evident within the EA₄ Pb₃ Br₁₀ SCFs through systematic characterizations. Flexible photodetectors based on EA₄ Pb₃ Br₁₀ SCFs are achieved with an impressive photodetection performance. More importantly, optoelectronic EA₄ Pb₃ Br₁₀ SCFs incorporated with in-plane ferroelectric polarization and effective piezoelectric coefficient show great promise for the observation of piezo-phototronic effect, which is capable of greatly enhancing the photodetector performance. Under external strains, the responsivity of the flexible photodetectors can be modulated by piezo-phototronic effect with a remarkable enhancement up to 284%. Our findings shed light on the piezo-phototronic devices and offer a promising avenue to broaden functionalities of hybrid perovskite ferroelectrics.

PFM (piezoresponse force microscopy)-aided design for molecular ferroelectrics

With prosperity, decay, and another spring, molecular ferroelectrics have passed a hundred years since Valasek first discovered ferroelectricity in the molecular compound Rochelle salt. Recently, the proposal of ferroelectrochemistry has injected new vigor into this century-old research field. It should be highlighted that piezoresponse force microscopy (PFM) technique, as a non-destructive imaging and manipulation method for ferroelectric domains at the nanoscale, can significantly speed up the design rate of molecular ferroelectrics as well as enhance the ferroelectric and piezoelectric performances relying on domain engineering. Herein, we provide a brief review of the contribution of the PFM technique toward assisting the design and performance optimization of molecular ferroelectrics. Relying on the relationship between ferroelectric domains and crystallography, together with other physical characteristics such as domain switching and piezoelectricity, we believe that the PFM technique can be effectively applied to assist the design of high-performance molecular ferroelectrics equipped with multifunctionality, and thereby facilitate their practical utilization in optics, electronics, magnetics, thermotics, and mechanics among others.

Giant improvement of performances of perovskite solar cells via component engineering

The absorption layer is a crucial factor for high-performance perovskite solar cells. In this work, the influence of the two components, methylammonium iodide (MAI) and formamidinium iodide (FAI) on the morphology, optical absorption and photovoltaic performances was systematically investigated. The results revealed that the surface morphologies of MAI/FAI based perovskite films were rougher, and the grain sizes became larger with increasing the FAI concentration. UV-Vis and photoluminescence spectra showed that there was a red shift with enhancing the FAI concentration. By the effective doping of FAI into the pristine MAI based perovskite film, the formation of a δ-FAPbI₃ was successfully inhibited. As a result, the power conversion efficiency (PCE) of the perovskite solar cells based on mixed absorption layers was improved by about 27% compared to the pristine MAI based perovskite device.

Multimodal noncontact atomic force microscopy and Kelvin probe force microscopy investigations of organolead tribromide perovskite single crystals

In this work, methylammonium lead tribromide (MAPbBr₃) single crystals are studied by noncontact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM). We demonstrate that the surface photovoltage and crystal photostriction can be simultaneously investigated by implementing a specific protocol based on the acquisition of the tip height and surface potential during illumination sequences. The obtained data confirm the existence of lattice expansion under illumination in MAPbBr₃ and that negative photocarriers accumulate near the crystal surface due to band bending effects. Time-dependent changes of the surface potential occurring under illumination on the scale of a few seconds reveal the existence of slow ion-migration mechanisms. Lastly, photopotential decay at the sub-millisecond time scale related to the photocarrier lifetime is quantified by performing KPFM measurements under frequency-modulated illumination. Our multimodal approach provides a unique way to investigate the interplay between the charges and ionic species, the photocarrier-lattice coupling and the photocarrier dynamics in hybrid perovskites.

Multinuclear NMR as a tool for studying local order and dynamics in CH3NH3PbX3 (X = Cl, Br, I) hybrid perovskites

We report on ²⁰⁷Pb, ⁷⁹Br, ¹⁴N, ¹H, ¹³C and ²H NMR experiments for studying the local order and dynamics in hybrid perovskite lattices. ²⁰⁷Pb NMR experiments conducted at room temperature on a series of MAPbX₃ compounds (MA = CH₃NH₃⁺; X = Cl, Br and I) showed that the isotropic ²⁰⁷Pb NMR shift is strongly dependent on the nature of the halogen ions. Therefore ²⁰⁷Pb NMR appears to be a very promising tool for the characterisation of local order in mixed halogen hybrid perovskites. ²⁰⁷Pb NMR on MAPbBr₂I served as a proof of concept. Proton, ¹³C and ¹⁴N NMR experiments confirmed the results previously reported in the literature. Low temperature deuterium NMR measurements, down to 25 K, were carried out to investigate the structural phase transitions of MAPbBr₃. Spectral lineshapes allow following the successive phase transitions of MAPbBr₃. Finally, quadrupolar NMR lineshapes recorded in the orthorhombic phase were compared with simulated spectra, using DFT calculated electric field gradients (EFG). Computed data do not take into account any temperature effect. Thus, the discrepancy between the calculated and experimental EFG evidences the fact that MA cations are still subject to significant dynamics, even at 25 K.

Recent Progress in Single-Crystalline Perovskite Research Including Crystal Preparation, Property Evaluation, and Applications

Organic-inorganic lead halide perovskites are promising optoelectronic materials resulting from their significant light absorption properties and unique long carrier dynamics, such as a long carrier lifetime, carrier diffusion length, and high carrier mobility. These advantageous properties have allowed for the utilization of lead halide perovskite materials in solar cells, LEDs, photodetectors, lasers, etc. To further explore their potential, intrinsic properties should be thoroughly investigated. Single crystals with few defects are the best candidates to disclose a variety of interesting and important properties of these materials, ultimately, showing the increased importance of single-crystalline perovskite research. In this review, recent progress on the crystallization, investigation, and primary device applications of single-crystalline perovskites are summarized and analyzed. Further improvements in device design and preparation are also discussed.

Growth of centimeter-sized [(CH3)2NH2][Mn(HCOO)3] hybrid formate perovskite single crystals and Raman evidence of pressure-induced phase transitions

The fewer the number of the nucleation sites formed in the vessel, the larger the size of the obtained crystals.

Giant Rashba Splitting in CH_{3}NH_{3}PbBr_{3} Organic-Inorganic Perovskite

As they combine decent mobilities with extremely long carrier lifetimes, organic-inorganic perovskites open a whole new field in optoelectronics. Measurements of their underlying electronic structure, however, are still lacking. Using angle-resolved photoelectron spectroscopy, we measure the valence band dispersion of single-crystal CH_{3}NH_{3}PbBr_{3}. The dispersion of the highest energy band is extracted applying a modified leading edge method, which accounts for the particular density of states of organic-inorganic perovskites. The surface Brillouin zone is consistent with bulk-terminated surfaces both in the low-temperature orthorhombic and the high-temperature cubic phase. In the low-temperature phase, we find a ring-shaped valence band maximum with a radius of 0.043  Å^{-1}, centered around a 0.16 eV deep local minimum in the dispersion of the valence band at the high-symmetry point. Intense circular dichroism is observed. This dispersion is the result of strong spin-orbit coupling. Spin-orbit coupling is also present in the room-temperature phase. The coupling strength is one of the largest ones reported so far.

High Efficiency Pb-In Binary Metal Perovskite Solar Cells

Mixed Pb-In perovskite solar cells are fabricated by using lead(II) chloride and indium(III) chloride with methylammonium iodide. A maximum power conversion efficiency as high as 17.55% is achieved owing to the high quality of perovskites with multiple ordered crystal orientations.

Structure and Growth Control of Organic-Inorganic Halide Perovskites for Optoelectronics: From Polycrystalline Films to Single Crystals

Recently, organic-inorganic halide perovskites have sparked tremendous research interest because of their ground-breaking photovoltaic performance. The crystallization process and crystal shape of perovskites have striking impacts on their optoelectronic properties. Polycrystalline films and single crystals are two main forms of perovskites. Currently, perovskite thin films have been under intensive investigation while studies of perovskite single crystals are just in their infancy. This review article is concentrated upon the control of perovskite structures and growth, which are intimately correlated for improvements of not only solar cells but also light-emitting diodes, lasers, and photodetectors. We begin with the survey of the film formation process of perovskites including deposition methods and morphological optimization avenues. Strategies such as the use of additives, thermal annealing, solvent annealing, atmospheric control, and solvent engineering have been successfully employed to yield high-quality perovskite films. Next, we turn to summarize the shape evolution of perovskites single crystals from three-dimensional large sized single crystals, two-dimensional nanoplates, one-dimensional nanowires, to zero-dimensional quantum dots. Siginificant functions of perovskites single crystals are highlighted, which benefit fundamental studies of intrinsic photophysics. Then, the growth mechanisms of the previously mentioned perovskite crystals are unveiled. Lastly, perspectives for structure and growth control of perovskites are outlined towards high-performance (opto)electronic devices.